This invention relates to turbo-machinery and more particularly to film cooled airfoil bodies associated with a gas turbine engine.
It has long been recognized that airfoils, such as turbine blades and vanes operating in the high temperature gas environment of a gas turbine engine, must be cooled by a fluid medium in order to achieve favorable life performance and hence low maintenance and overhaul expense on the engine. It has become common practice to divert air from the compressor section of the engine to a cavity within the airfoil and thence through a series of passages through the walls of the airfoil. Air emerging from the passages is intended to establish a moving film of cooling air between the surface of the airfoil and the hot gases flowing within the engine. The moving film of cooling air is intended to form a barrier between the hot gases and the airfoil surface and hence to inhibit the transfer of heat from the gases to the airfoil surface. The life of the airfoil is consequently enhanced.
The cooling passages associated with prior art devices have been disposed widely spaced-apart from each other over the surface of the airfoil. Thus, the cooling air emerges from the cooling passages in the form of a plurality of discrete jets separated by wakes void of cooling air. Hence, elongated hot streaks appear in the area of the wake where the hot gases flowing in the engine may contact the surface of the airfoil. The airfoil surface is subjected to alternating hot and cool areas which result in the establishment of thermal stress gradients and excessively hot spots and an associated decrease in airfoil life. Some prior art cooling schemes have addressed these shortcomings. By way of example, the device shown in U.S. Pat. No. 3,527,543 discloses the use of diverging cooling passages to provide for the discharge of discrete jets of cooling air that converge toward each other. However, in the device shown in this reference, the spacing between cooling passages at their exit is substantially greater than the width of the passages. Hence, even though the cooling passages are tapered the wakes are not eliminated and the airfoil still suffers from the aforementioned shortcomings. The present invention addresses the elimination of wakes associated with prior art airfoils having a plurality of cooling air passages providing a film of cooling air to the airfoil surface.
Therefore, it is an object of the present invention to provide for a film cooled airfoil wherein the cooling air is introduced over the airfoil surface in the form of a continuous film.